The present invention relates to arrangements for restoring storage capacity of a vehicle emission control device.
Generally, lean burn engines in vehicles generate tailpipe NOx when operating in a normal lean burn cycle, i.e., operating the engine with an air-fuel ratio (AFR) leaner than stoichiometric. Vehicle exhaust treatment systems utilize a three-way catalyst (TWC), referred to as an emission control device or lean Nox trap, positioned in an exhaust passage to store and release constituent gases such as NOx depending on engine operating conditions.
More specifically, storage capacity of such emission control devices is limited. As a result, stored gas is released by periodically operating the engine in a rich AFR cycle. In addition, various factors such as the type of fuel and overall aging of the emission control device act to reduce original storage capacity.
Generally, storage capacity of the emission control device can be potentially restored by periodically operating the engine in such a manner as to raise the temperature of the exhaust to a predetermined temperature range. As part of this process, the AFR must be controlled to properly sustain the desired temperature range. As a consequence, a need exists for an arrangement capable of accurately controlling temperature within an emission control device when attempting to restore storage capacity.
Therefore, it is an object of the present invention to provide a temperature control system and method which utilizes a closed-loop arrangement to dynamically generate an estimate for temperature within an emission control device to produce an AFR correction factor which will maintain the temperature within a desired range.
In accordance with a first aspect of the present invention, a system is provided for controlling the temperature within an emission control device, the emission control device having a catalyst bed for reducing emission of a constituent gas in an exhaust system of an engine, wherein the system includes a first temperature sensor connected to the emission control device to generate an output representative of temperature within the emission control device, and a control processor coupled to a fuel control subsystem and arranged to estimate actual temperature within the catalyst bed based on the output of the first temperature sensor. The control processor is further arranged to compare the estimated emission control device temperature with a desired temperature, and control the air-fuel ratio as a function of the comparison to control the temperature within the emission control device.
In accordance with a second aspect of the present invention, a method is provided for controlling the temperature within an emission control device, the emission control device having a catalyst bed for reducing emission of a constituent gas in an exhaust system of an engine, wherein the method includes measuring the temperature of exhaust gases flowing through the emission control device, and determining an estimate of actual temperature at the catalyst bed based on the output of the temperature measurement. The estimated emission control device temperature is then compared with a desired temperature, and an air-fuel ratio of the exhaust gases is modified based on the temperature comparison to control the temperature within the emission control device.
In a preferred embodiment, the first temperature sensor is positioned at a midbrick location in the emission control device to provide a direct and accurate measurement of temperature within the device. In addition, the inlet temperature can be either directly measured or estimated from a temperature model as a function of monitored engine operating conditions.
Thus, because the present invention provides continual and accurate monitoring of the temperature in the emission control device along with corresponding AFR adjustments, the present invention assures accurate temperature control to minimize the possibility of damage to the emission control device due to excessive lean AFR when the device is hot, while also maximizing the potential for restoring storage capacity with a rich AFR when the trap is at a desired temperature. Accordingly, improvement is attained in the efficiency of the overall operation of the lean burn exhaust treatment system.